Method and apparatus for adjusting a blanking period for selecting a sensing vector configuration in a medical device

ABSTRACT

A method and medical device for determining sensing vectors that includes sensing cardiac signals from a plurality of electrodes, the plurality of electrodes forming a plurality of sensing vectors, setting a blanking period and a blanking period adjustment window for the plurality of sensing vectors in response to the sensed cardiac signals, determining first signal differences during the blanking period adjustment window, and adjusting the blanking period in response to the determined first signal differences.

RELATED APPLICATION

The present application claims priority and other benefits from U.S.Provisional Patent Application Ser. No. 61/983,499, filed Apr. 24, 2014,entitled “METHOD AND APPARATUS FOR SELECTING A SENSING VECTORCONFIGURATION IN A MEDICAL DEVICE”, incorporated herein by reference inits entirety.

TECHNICAL FIELD

The disclosure relates generally to implantable medical devices and, inparticular, to an apparatus and method for selecting a sensing vector ina medical device.

BACKGROUND

Implantable medical devices are available for preventing or treatingcardiac arrhythmias by delivering anti-tachycardia pacing therapies andelectrical shock therapies for cardioverting or defibrillating theheart. Such a device, commonly known as an implantable cardioverterdefibrillator or “ICD”, senses a patient's heart rhythm and classifiesthe rhythm according to a number of rate zones in order to detectepisodes of tachycardia or fibrillation.

Upon detecting an abnormal rhythm, the ICD delivers an appropriatetherapy. Pathologic forms of ventricular tachycardia can often beterminated by anti-tachycardia pacing therapies. Anti-tachycardia pacingtherapies are followed by high-energy shock therapy when necessary.Termination of a tachycardia by a shock therapy is commonly referred toas “cardioversion.” Ventricular fibrillation (VF) is a form oftachycardia that is a serious life-threatening condition and is normallytreated by immediately delivering high-energy shock therapy. Terminationof VF is commonly referred to as “defibrillation.” Accurate arrhythmiadetection and discrimination are important in selecting the appropriatetherapy for effectively treating an arrhythmia and avoiding the deliveryof unnecessary cardioversion/defibrillation (CV/DF) shocks, which arepainful to the patient.

In past practice, ICD systems have employed intra-cardiac electrodescarried by transvenous leads for sensing cardiac electrical signals anddelivering electrical therapies. Emerging ICD systems are adapted forsubcutaneous or submuscular implantation and employ electrodesincorporated on the ICD housing and/or carried by subcutaneous orsubmuscular leads. These systems, referred to generally herein as“subcutaneous ICD” or “SubQ ICD” systems, do not rely on electrodesimplanted in direct contact with the heart. SubQ ICD systems are lessinvasive and are therefore implanted more easily and quickly than ICDsystems that employ intra-cardiac electrodes. However, greaterchallenges exist in reliably detecting cardiac arrhythmias using asubcutaneous system. The R-wave amplitude on a SubQ ECG signal may be onthe order of one-tenth to one-one hundredth of the amplitude ofintra-ventricular sensed R-waves. Furthermore, the signal quality ofsubcutaneously sensed ECG signals are likely to be more affected bymyopotential noise, environmental noise, patient posture and patientactivity than intra-cardiac myocardial electrogram (EGM) signals.

The ability of a subcutaneous ICD to detect tachyarrhythmias and rejectnoise depends on its ECG signal characteristics. ECG vectors with higheramplitude R-wave waves, higher frequency (high slew rate) R-waves,higher R/T wave ratios, lower frequency signal (e.g., P and T waves)around R-waves, lower susceptibility to skeletal myopotentials, andgreater R-wave consistency from cycle to cycle are preferred to ECGvectors without these attributes. A subcutaneous ICD with a minimum of 2ECG leads or vectors (using a minimum of 3 electrodes) in a plane mayuse these physical vectors to generate virtual ECG vectors using alinear combination of the physical vector ECGs. However, choosing theoptimal vector may sometimes be a challenge given the changingenvironment of a subcutaneous system. As such, systems and methods thatpromote reliable and accurate sensing detection of arrhythmias usingoptimal available sensing vectors to sense ECG signals via subcutaneouselectrodes are needed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual diagram of a patient implanted with an exampleextravascular cardiac defibrillation system.

FIG. 2 is an exemplary schematic diagram of electronic circuitry withina hermetically sealed housing of a subcutaneous device according to anembodiment of the present invention.

FIG. 3 is a graphical representation of cardiac signals sensed alongmultiple sensing vectors during selection of a sensing vector in amedical device according to one embodiment.

FIG. 4 is a flowchart of a method for selecting one or more sensingvectors according to an exemplary embodiment.

FIG. 5 is a flowchart of a method for selecting one or more sensingvectors according to another exemplary embodiment.

FIG. 6 is a flowchart of a method for selecting one or more sensingvectors according to another exemplary embodiment.

DETAILED DESCRIPTION

FIG. 1 is a conceptual diagram of a patient 12 implanted with an exampleextravascular cardiac defibrillation system 10. In the exampleillustrated in FIG. 1, extravascular cardiac defibrillation system 10 isan implanted subcutaneous ICD system. However, the techniques of thisdisclosure may also be utilized with other extravascular implantedcardiac defibrillation systems, such as a cardiac defibrillation systemhaving a lead implanted at least partially in a substernal orsubmuscular location. Additionally, the techniques of this disclosuremay also be utilized with other implantable systems, such as implantablepacing systems, implantable neurostimulation systems, drug deliverysystems or other systems in which leads, catheters or other componentsare implanted at extravascular locations within patient 12. Thisdisclosure, however, is described in the context of an implantableextravascular cardiac defibrillation system for purposes ofillustration.

Extravascular cardiac defibrillation system 10 includes an implantablecardioverter defibrillator (ICD) 14 connected to at least oneimplantable cardiac defibrillation lead 16. ICD 14 of FIG. 1 isimplanted subcutaneously on the left side of patient 12. Defibrillationlead 16, which is connected to ICD 14, extends medially from ICD 14toward sternum 28 and xiphoid process 24 of patient 12. At a locationnear xiphoid process 24, defibrillation lead 16 bends or turns andextends subcutaneously superior, substantially parallel to sternum 28.In the example illustrated in FIG. 1, defibrillation lead 16 isimplanted such that lead 16 is offset laterally to the left side of thebody of sternum 28 (i.e., towards the left side of patient 12).

Defibrillation lead 16 is placed along sternum 28 such that a therapyvector between defibrillation electrode 18 and a second electrode (suchas a housing or can 25 of ICD 14 or an electrode placed on a secondlead) is substantially across the ventricle of heart 26. The therapyvector may, in one example, be viewed as a line that extends from apoint on the defibrillation electrode 18 to a point on the housing orcan 25 of ICD 14. In another example, defibrillation lead 16 may beplaced along sternum 28 such that a therapy vector betweendefibrillation electrode 18 and the housing or can 25 of ICD 14 (orother electrode) is substantially across an atrium of heart 26. In thiscase, extravascular ICD system 10 may be used to provide atrialtherapies, such as therapies to treat atrial fibrillation.

The embodiment illustrated in FIG. 1 is an example configuration of anextravascular ICD system 10 and should not be considered limiting of thetechniques described herein. For example, although illustrated as beingoffset laterally from the midline of sternum 28 in the example of FIG.1, defibrillation lead 16 may be implanted such that lead 16 is offsetto the right of sternum 28 or more centrally located over sternum 28.Additionally, defibrillation lead 16 may be implanted such that it isnot substantially parallel to sternum 28, but instead offset fromsternum 28 at an angle (e.g., angled lateral from sternum 28 at eitherthe proximal or distal end). As another example, the distal end ofdefibrillation lead 16 may be positioned near the second or third rib ofpatient 12. However, the distal end of defibrillation lead 16 may bepositioned further superior or inferior depending on the location of ICD14, location of electrodes 18, 20, and 22, or other factors.

Although ICD 14 is illustrated as being implanted near a midaxillaryline of patient 12, ICD 14 may also be implanted at other subcutaneouslocations on patient 12, such as further posterior on the torso towardthe posterior axillary line, further anterior on the torso toward theanterior axillary line, in a pectoral region, or at other locations ofpatient 12. In instances in which ICD 14 is implanted pectorally, lead16 would follow a different path, e.g., across the upper chest area andinferior along sternum 28. When the ICD 14 is implanted in the pectoralregion, the extravascular ICD system may include a second lead includinga defibrillation electrode that extends along the left side of thepatient such that the defibrillation electrode of the second lead islocated along the left side of the patient to function as an anode orcathode of the therapy vector of such an ICD system.

ICD 14 includes a housing or can 25 that forms a hermetic seal thatprotects components within ICD 14. The housing 25 of ICD 14 may beformed of a conductive material, such as titanium or other biocompatibleconductive material or a combination of conductive and non-conductivematerials. In some instances, the housing 25 of ICD 14 functions as anelectrode (referred to as a housing electrode or can electrode) that isused in combination with one of electrodes 18, 20, or 22 to deliver atherapy to heart 26 or to sense electrical activity of heart 26. ICD 14may also include a connector assembly (sometimes referred to as aconnector block or header) that includes electrical feedthroughs throughwhich electrical connections are made between conductors withindefibrillation lead 16 and electronic components included within thehousing. Housing may enclose one or more components, includingprocessors, memories, transmitters, receivers, sensors, sensingcircuitry, therapy circuitry and other appropriate components (oftenreferred to herein as modules).

Defibrillation lead 16 includes a lead body having a proximal end thatincludes a connector configured to connect to ICD 14 and a distal endthat includes one or more electrodes 18, 20, and 22. The lead body ofdefibrillation lead 16 may be formed from a non-conductive material,including silicone, polyurethane, fluoropolymers, mixtures thereof, andother appropriate materials, and shaped to form one or more lumenswithin which the one or more conductors extend. However, the techniquesare not limited to such constructions. Although defibrillation lead 16is illustrated as including three electrodes 18, 20 and 22,defibrillation lead 16 may include more or fewer electrodes.

Defibrillation lead 16 includes one or more elongated electricalconductors (not illustrated) that extend within the lead body from theconnector on the proximal end of defibrillation lead 16 to electrodes18, 20 and 22. In other words, each of the one or more elongatedelectrical conductors contained within the lead body of defibrillationlead 16 may engage with respective ones of electrodes 18, 20 and 22.When the connector at the proximal end of defibrillation lead 16 isconnected to ICD 14, the respective conductors may electrically coupleto circuitry, such as a therapy module or a sensing module, of ICD 14via connections in connector assembly, including associatedfeedthroughs. The electrical conductors transmit therapy from a therapymodule within ICD 14 to one or more of electrodes 18, 20 and 22 andtransmit sensed electrical signals from one or more of electrodes 18, 20and 22 to the sensing module within ICD 14.

ICD 14 may sense electrical activity of heart 26 via one or more sensingvectors that include combinations of electrodes 20 and 22 and thehousing or can 25 of ICD 14. For example, ICD 14 may obtain electricalsignals sensed using a sensing vector between electrodes 20 and 22,obtain electrical signals sensed using a sensing vector betweenelectrode 20 and the conductive housing or can 25 of ICD 14, obtainelectrical signals sensed using a sensing vector between electrode 22and the conductive housing or can 25 of ICD 14, or a combinationthereof. In some instances, ICD 14 may sense cardiac electrical signalsusing a sensing vector that includes defibrillation electrode 18, suchas a sensing vector between defibrillation electrode 18 and one ofelectrodes 20 or 22, or a sensing vector between defibrillationelectrode 18 and the housing or can 25 of ICD 14.

ICD may analyze the sensed electrical signals to detect tachycardia,such as ventricular tachycardia or ventricular fibrillation, and inresponse to detecting tachycardia may generate and deliver an electricaltherapy to heart 26. For example, ICD 14 may deliver one or moredefibrillation shocks via a therapy vector that includes defibrillationelectrode 18 of defibrillation lead 16 and the housing or can 25.Defibrillation electrode 18 may, for example, be an elongated coilelectrode or other type of electrode. In some instances, ICD 14 maydeliver one or more pacing therapies prior to or after delivery of thedefibrillation shock, such as anti-tachycardia pacing (ATP) or postshock pacing. In these instances, ICD 14 may generate and deliver pacingpulses via therapy vectors that include one or both of electrodes 20 and22 and/or the housing or can 25. Electrodes 20 and 22 may comprise ringelectrodes, hemispherical electrodes, coil electrodes, helix electrodes,segmented electrodes, directional electrodes, or other types ofelectrodes, or combination thereof. Electrodes 20 and 22 may be the sametype of electrodes or different types of electrodes, although in theexample of FIG. 1 both electrodes 20 and 22 are illustrated as ringelectrodes.

Defibrillation lead 16 may also include an attachment feature 29 at ortoward the distal end of lead 16. The attachment feature 29 may be aloop, link, or other attachment feature. For example, attachment feature29 may be a loop formed by a suture. As another example, attachmentfeature 29 may be a loop, link, ring of metal, coated metal or apolymer. The attachment feature 29 may be formed into any of a number ofshapes with uniform or varying thickness and varying dimensions.Attachment feature 29 may be integral to the lead or may be added by theuser prior to implantation. Attachment feature 29 may be useful to aidin implantation of lead 16 and/or for securing lead 16 to a desiredimplant location. In some instances, defibrillation lead 16 may includea fixation mechanism in addition to or instead of the attachmentfeature. Although defibrillation lead 16 is illustrated with anattachment feature 29, in other examples lead 16 may not include anattachment feature 29.

Lead 16 may also include a connector at the proximal end of lead 16,such as a DF4 connector, bifurcated connector (e.g., DF-1/IS-1connector), or other type of connector. The connector at the proximalend of lead 16 may include a terminal pin that couples to a port withinthe connector assembly of ICD 14. In some instances, lead 16 may includean attachment feature at the proximal end of lead 16 that may be coupledto an implant tool to aid in implantation of lead 16. The attachmentfeature at the proximal end of the lead may separate from the connectorand may be either integral to the lead or added by the user prior toimplantation.

Defibrillation lead 16 may also include a suture sleeve or otherfixation mechanism (not shown) located proximal to electrode 22 that isconfigured to fixate lead 16 near the xiphoid process or lower sternumlocation. The fixation mechanism (e.g., suture sleeve or othermechanism) may be integral to the lead or may be added by the user priorto implantation.

The example illustrated in FIG. 1 is exemplary in nature and should notbe considered limiting of the techniques described in this disclosure.For instance, extravascular cardiac defibrillation system 10 may includemore than one lead. In one example, extravascular cardiac defibrillationsystem 10 may include a pacing lead in addition to defibrillation lead16.

In the example illustrated in FIG. 1, defibrillation lead 16 isimplanted subcutaneously, e.g., between the skin and the ribs orsternum. In other instances, defibrillation lead 16 (and/or the optionalpacing lead) may be implanted at other extravascular locations. In oneexample, defibrillation lead 16 may be implanted at least partially in asubsternal location. In such a configuration, at least a portion ofdefibrillation lead 16 may be placed under or below the sternum in themediastinum and, more particularly, in the anterior mediastinum. Theanterior mediastinum is bounded laterally by pleurae, posteriorly bypericardium, and anteriorly by sternum 28. Defibrillation lead 16 may beat least partially implanted in other extra-pericardial locations, i.e.,locations in the region around, but not in direct contact with, theouter surface of heart 26. These other extra-pericardial locations mayinclude in the mediastinum but offset from sternum 28, in the superiormediastinum, in the middle mediastinum, in the posterior mediastinum, inthe sub-xiphoid or inferior xiphoid area, near the apex of the heart, orother location not in direct contact with heart 26 and not subcutaneous.In still further instances, the lead may be implanted at a pericardialor epicardial location outside of the heart 26.

FIG. 2 is an exemplary schematic diagram of electronic circuitry withina hermetically sealed housing of a subcutaneous device according to anembodiment of the present invention. As illustrated in FIG. 2,subcutaneous device 14 includes a low voltage battery 153 coupled to apower supply (not shown) that supplies power to the circuitry of thesubcutaneous device 14 and the pacing output capacitors to supply pacingenergy in a manner well known in the art. The low voltage battery 153may be formed of one or two conventional LiCF_(x), LiMnO₂ or Lilt cells,for example. The subcutaneous device 14 also includes a high voltagebattery 112 that may be formed of one or two conventional LiSVO orLiMnO₂ cells. Although two both low voltage battery and a high voltagebattery are shown in FIG. 2, according to an embodiment of the presentinvention, the device 14 could utilize a single battery for both highand low voltage uses.

Further referring to FIG. 2, subcutaneous device 14 functions arecontrolled by means of software, firmware and hardware thatcooperatively monitor the ECG signal, determine when acardioversion-defibrillation shock or pacing is necessary, and deliverprescribed cardioversion-defibrillation and pacing therapies. Thesubcutaneous device 14 may incorporate circuitry set forth in commonlyassigned U.S. Pat. Nos. 5,163,427 “Apparatus for Delivering Single andMultiple Cardioversion and Defibrillation Pulses” to Keimel and5,188,105 “Apparatus and Method for Treating a Tachyarrhythmia” toKeimel for selectively delivering single phase, simultaneous biphasicand sequential biphasic cardioversion-defibrillation shocks typicallyemploying ICD IPG housing electrodes 28 coupled to the COMMON output 123of high voltage output circuit 140 and cardioversion-defibrillationelectrode 24 disposed posterially and subcutaneously and coupled to theHVI output 113 of the high voltage output circuit 140.

The cardioversion-defibrillation shock energy and capacitor chargevoltages can be intermediate to those supplied by ICDs having at leastone cardioversion-defibrillation electrode in contact with the heart andmost AEDs having cardioversion-defibrillation electrodes in contact withthe skin. The typical maximum voltage necessary for ICDs using mostbiphasic waveforms is approximately 750 Volts with an associated maximumenergy of approximately 40 Joules. The typical maximum voltage necessaryfor AEDs is approximately 2000-5000 Volts with an associated maximumenergy of approximately 200-360 Joules depending upon the model andwaveform used. The subcutaneous device 14 of the present invention usesmaximum voltages in the range of about 300 to approximately 1500 Voltsand is associated with energies of approximately 25 to 150 joules ormore. The total high voltage capacitance could range from about 50 toabout 300 microfarads. Such cardioversion-defibrillation shocks are onlydelivered when a malignant tachyarrhythmia, e.g., ventricularfibrillation is detected through processing of the far field cardiac ECGemploying the detection algorithms as described herein below.

In FIG. 2, sense amp 190 in conjunction with pacer/device timing circuit178 processes the far field ECG sense signal that is developed across aparticular ECG sense vector defined by a selected pair of thesubcutaneous electrodes 18, 20, 22 and the can or housing 25 of thedevice 14, or, optionally, a virtual signal (i.e., a mathematicalcombination of two vectors) if selected. For example, the device maygenerate a virtual vector signal as described in U.S. Pat. No. 6,505,067“System and Method for Deriving Virtual ECG or EGM Signal” to Lee, etal; incorporated herein by reference in it's entirety. In addition,vector selection may be selected by the patient's physician andprogrammed via a telemetry link from a programmer.

The selection of the sensing electrode pair is made through the switchmatrix/MUX 191 in a manner to provide the most reliable sensing of theECG signal of interest, which would be the R wave for patients who arebelieved to be at risk of ventricular fibrillation leading to suddendeath. The far field ECG signals are passed through the switchmatrix/MUX 191 to the input of the sense amplifier 190 that, inconjunction with pacer/device timing circuit 178, evaluates the sensedECG. Bradycardia, or asystole, is typically determined by an escapeinterval timer within the pacer timing circuit 178 and/or the controlcircuit 144. Pace Trigger signals are applied to the pacing pulsegenerator 192 generating pacing stimulation when the interval betweensuccessive R-waves exceeds the escape interval. Bradycardia pacing isoften temporarily provided to maintain cardiac output after delivery ofa cardioversion-defibrillation shock that may cause the heart to slowlybeat as it recovers back to normal function. Sensing subcutaneous farfield signals in the presence of noise may be aided by the use ofappropriate denial and extensible accommodation periods as described inU.S. Pat. No. 6,236,882 “Noise Rejection for Monitoring ECGs” to Lee, etal and incorporated herein by reference in its' entirety.

Detection of a malignant tachyarrhythmia is determined in the Controlcircuit 144 as a function of the intervals between R-wave sense eventsignals that are output from the pacer/device timing 178 and senseamplifier circuit 190 to the timing and control circuit 144. It shouldbe noted that the present invention utilizes not only interval basedsignal analysis method but also supplemental sensors and morphologyprocessing method and apparatus as described herein below.

Supplemental sensors such as tissue color, tissue oxygenation,respiration, patient activity and the like may be used to contribute tothe decision to apply or withhold a defibrillation therapy as describedgenerally in U.S. Pat. No. 5,464,434 “Medical Interventional DeviceResponsive to Sudden Hemodynamic Change” to Alt and incorporated hereinby reference in its entirety. Sensor processing block 194 providessensor data to microprocessor 142 via data bus 146. Specifically,patient activity and/or posture may be determined by the apparatus andmethod as described in U.S. Pat. No. 5,593,431 “Medical ServiceEmploying Multiple DC Accelerometers for Patient Activity and PostureSensing and Method” to Sheldon and incorporated herein by reference inits entirety. Patient respiration may be determined by the apparatus andmethod as described in U.S. Pat. No. 4,567,892 “Implantable CardiacPacemaker” to Plicchi, et al and incorporated herein by reference in itsentirety. Patient tissue oxygenation or tissue color may be determinedby the sensor apparatus and method as described in U.S. Pat. No.5,176,137 to Erickson, et al and incorporated herein by reference in itsentirety. The oxygen sensor of the '137 patent may be located in thesubcutaneous device pocket or, alternatively, located on the lead 18 toenable the sensing of contacting or near-contacting tissue oxygenationor color.

Certain steps in the performance of the detection algorithm criteria arecooperatively performed in microcomputer 142, including microprocessor,RAM and ROM, associated circuitry, and stored detection criteria thatmay be programmed into RAM via a telemetry interface (not shown)conventional in the art. Data and commands are exchanged betweenmicrocomputer 142 and timing and control circuit 144, pacertiming/amplifier circuit 178, and high voltage output circuit 140 via abi-directional data/control bus 146. The pacer timing/amplifier circuit178 and the control circuit 144 are clocked at a slow clock rate. Themicrocomputer 142 is normally asleep, but is awakened and operated by afast clock by interrupts developed by each R-wave sense event, onreceipt of a downlink telemetry programming instruction or upon deliveryof cardiac pacing pulses to perform any necessary mathematicalcalculations, to perform tachycardia and fibrillation detectionprocedures, and to update the time intervals monitored and controlled bythe timers in pacer/device timing circuitry 178.

When a malignant tachycardia is detected, high voltage capacitors 156,158, 160, and 162 are charged to a pre-programmed voltage level by ahigh-voltage charging circuit 164. It is generally consideredinefficient to maintain a constant charge on the high voltage outputcapacitors 156, 158, 160, 162. Instead, charging is initiated whencontrol circuit 144 issues a high voltage charge command HVCHG deliveredon line 145 to high voltage charge circuit 164 and charging iscontrolled by means of bi-directional control/data bus 166 and afeedback signal VCAP from the HV output circuit 140. High voltage outputcapacitors 156, 158, 160 and 162 may be of film, aluminum electrolyticor wet tantalum construction.

The negative terminal of high voltage battery 112 is directly coupled tosystem ground. Switch circuit 114 is normally open so that the positiveterminal of high voltage battery 112 is disconnected from the positivepower input of the high voltage charge circuit 164. The high voltagecharge command HVCHG is also conducted via conductor 149 to the controlinput of switch circuit 114, and switch circuit 114 closes in responseto connect positive high voltage battery voltage EXT B+ to the positivepower input of high voltage charge circuit 164. Switch circuit 114 maybe, for example, a field effect transistor (FET) with itssource-to-drain path interrupting the EXT B+conductor 118 and its gatereceiving the HVCHG signal on conductor 145. High voltage charge circuit164 is thereby rendered ready to begin charging the high voltage outputcapacitors 156, 158, 160, and 162 with charging current from highvoltage battery 112.

High voltage output capacitors 156, 158, 160, and 162 may be charged tovery high voltages, e.g., 300-1500V, to be discharged through the bodyand heart between the electrode pair of subcutaneouscardioversion-defibrillation electrodes 113 and 123. The details of thevoltage charging circuitry are also not deemed to be critical withregard to practicing the present invention; one high voltage chargingcircuit believed to be suitable for the purposes of the presentinvention is disclosed. High voltage capacitors 156, 158, 160 and 162may be charged, for example, by high voltage charge circuit 164 and ahigh frequency, high-voltage transformer 168 as described in detail incommonly assigned U.S. Pat. No. 4,548,209 “Energy Converter forImplantable Cardioverter” to Wielders, et al. Proper charging polaritiesare maintained by diodes 170, 172, 174 and 176 interconnecting theoutput windings of high-voltage transformer 168 and the capacitors 156,158, 160, and 162. As noted above, the state of capacitor charge ismonitored by circuitry within the high voltage output circuit 140 thatprovides a VCAP, feedback signal indicative of the voltage to the timingand control circuit 144. Timing and control circuit 144 terminates thehigh voltage charge command HVCHG when the VCAP signal matches theprogrammed capacitor output voltage, i.e., thecardioversion-defibrillation peak shock voltage.

Control circuit 144 then develops first and second control signalsNPULSE 1 and NPULSE 2, respectively, that are applied to the highvoltage output circuit 140 for triggering the delivery of cardiovertingor defibrillating shocks. In particular, the NPULSE 1 signal triggersdischarge of the first capacitor bank, comprising capacitors 156 and158. The NPULSE 2 signal triggers discharge of the first capacitor bankand a second capacitor bank, comprising capacitors 160 and 162. It ispossible to select between a plurality of output pulse regimes simply bymodifying the number and time order of assertion of the NPULSE 1 andNPULSE 2 signals. The NPULSE 1 signals and NPULSE 2 signals may beprovided sequentially, simultaneously or individually. In this way,control circuitry 144 serves to control operation of the high voltageoutput stage 140, which delivers high energycardioversion-defibrillation shocks between the pair of thecardioversion-defibrillation electrodes 18 and 25 coupled to the HV-1and COMMON output as shown in FIG. 2.

Thus, subcutaneous device 14 monitors the patient's cardiac status andinitiates the delivery of a cardioversion-defibrillation shock throughthe cardioversion-defibrillation electrodes 18 and 25 in response todetection of a tachyarrhythmia requiring cardioversion-defibrillation.The high HVCHG signal causes the high voltage battery 112 to beconnected through the switch circuit 114 with the high voltage chargecircuit 164 and the charging of output capacitors 156, 158, 160, and 162to commence. Charging continues until the programmed charge voltage isreflected by the VCAP signal, at which point control and timing circuit144 sets the HVCHG signal low terminating charging and opening switchcircuit 114. The subcutaneous device 14 can be programmed to attempt todeliver cardioversion shocks to the heart in the manners described abovein timed synchrony with a detected R-wave or can be programmed orfabricated to deliver defibrillation shocks to the heart in the mannersdescribed above without attempting to synchronize the delivery to adetected R-wave. Episode data related to the detection of thetachyarrhythmia and delivery of the cardioversion-defibrillation shockcan be stored in RAM for uplink telemetry transmission to an externalprogrammer as is well known in the art to facilitate in diagnosis of thepatient's cardiac state. A patient receiving the device 14 on aprophylactic basis would be instructed to report each such episode tothe attending physician for further evaluation of the patient'scondition and assessment for the need for implantation of a moresophisticated ICD.

Subcutaneous device 14 desirably includes telemetry circuit (not shownin FIG. 2), so that it is capable of being programmed by means ofexternal programmer 20 via a 2-way telemetry link (not shown). Uplinktelemetry allows device status and diagnostic/event data to be sent toexternal programmer 20 for review by the patient's physician. Downlinktelemetry allows the external programmer via physician control to allowthe programming of device function and the optimization of the detectionand therapy for a specific patient. Programmers and telemetry systemssuitable for use in the practice of the present invention have been wellknown for many years. Known programmers typically communicate with animplanted device via a bi-directional radio-frequency telemetry link, sothat the programmer can transmit control commands and operationalparameter values to be received by the implanted device, so that theimplanted device can communicate diagnostic and operational data to theprogrammer. Programmers believed to be suitable for the purposes ofpracticing the present invention include the Models 9790 and CareLink®programmers, commercially available from Medtronic, Inc., Minneapolis,Minn.

Various telemetry systems for providing the necessary communicationschannels between an external programming unit and an implanted devicehave been developed and are well known in the art. Telemetry systemsbelieved to be suitable for the purposes of practicing the presentinvention are disclosed, for example, in the following U.S. Patents:U.S. Pat. No. 5,127,404 to Wyborny et al. entitled “Telemetry Format forImplanted Medical Device”; U.S. Pat. No. 4,374,382 to Markowitz entitled“Marker Channel Telemetry System for a Medical Device”; and U.S. Pat.No. 4,556,063 to Thompson et al. entitled “Telemetry System for aMedical Device”. The Wyborny et al. '404, Markowitz '382, and Thompsonet al. '063 patents are commonly assigned to the assignee of the presentinvention, and are each hereby incorporated by reference herein in theirrespective entireties.

According to an embodiment of the present invention, in order toautomatically select the preferred ECG vector set, it is necessary tohave an index of merit upon which to rate the quality of the signal.“Quality” is defined as the signal's ability to provide accurate heartrate estimation and accurate morphological waveform separation betweenthe patient's usual sinus rhythm and the patient's ventriculartachyarrhythmia.

Appropriate indices may include R-wave amplitude, R-wave peak amplitudeto waveform amplitude between R-waves (i.e., signal to noise ratio), lowslope content, relative high versus low frequency power, mean frequencyestimation, probability density function, or some combination of thesemetrics.

Automatic vector selection might be done at implantation or periodically(daily, weekly, monthly) or both. At implant, automatic vector selectionmay be initiated as part of an automatic device turn-on procedure thatperforms such activities as measure lead impedances and batteryvoltages. The device turn-on procedure may be initiated by theimplanting physician (e.g., by pressing a programmer button) or,alternatively, may be initiated automatically upon automatic detectionof device/lead implantation. The turn-on procedure may also use theautomatic vector selection criteria to determine if ECG vector qualityis adequate for the current patient and for the device and leadposition, prior to suturing the subcutaneous device 14 device in placeand closing the incision. Such an ECG quality indicator would allow theimplanting physician to maneuver the device to a new location ororientation to improve the quality of the ECG signals as required. Thepreferred ECG vector or vectors may also be selected at implant as partof the device turn-on procedure. The preferred vectors might be thosevectors with the indices that maximize rate estimation and detectionaccuracy. There may also be an a priori set of vectors that arepreferred by the physician, and as long as those vectors exceed someminimum threshold, or are only slightly worse than some other moredesirable vectors, the a priori preferred vectors are chosen. Certainvectors may be considered nearly identical such that they are not testedunless the a priori selected vector index falls below some predeterminedthreshold.

Depending upon metric power consumption and power requirements of thedevice, the ECG signal quality metric may be measured on the range ofvectors (or alternatively, a subset) as often as desired. Data may begathered, for example, on a minute, hourly, daily, weekly or monthlybasis. More frequent measurements (e.g., every minute) may be averagedover time and used to select vectors based upon susceptibility ofvectors to occasional noise, motion noise, or EMI, for example.

Alternatively, the subcutaneous device 14 may have an indicator/sensorof patient activity (piezo-resistive, accelerometer, impedance, or thelike) and delay automatic vector measurement during periods of moderateor high patient activity to periods of minimal to no activity. Onerepresentative scenario may include testing/evaluating ECG vectors oncedaily or weekly while the patient has been determined to be asleep(using an internal clock (e.g., 2:00 am) or, alternatively, infer sleepby determining the patient's position (via a 2- or 3-axis accelerometer)and a lack of activity). In another possible scenario, thetesting/evaluating ECG vectors may be performed once daily or weeklywhile the patient is known to be exercising.

If infrequent automatic, periodic measurements are made, it may also bedesirable to measure noise (e.g., muscle, motion, EMI, etc.) in thesignal and postpone the vector selection measurement until a period oftime when the noise has subsided.

Subcutaneous device 14 may optionally have an indicator of the patient'sposture (via a 2- or 3-axis accelerometer). This sensor may be used toensure that the differences in ECG quality are not simply a result ofchanging posture/position. The sensor may be used to gather data in anumber of postures so that ECG quality may be averaged over thesepostures, or otherwise combined, or, alternatively, selected for apreferred posture.

In one embodiment, vector quality metric calculations may be performedby the clinician using a programmer either at the time of implant,during a subsequent visit in a clinic setting, or remotely via a remotelink with the device and the programmer. According to anotherembodiment, the vector quality metric calculations may be performedautomatically for each available sensing vector by the device apredetermined number of times, such multiple times daily, once per day,weekly or on a monthly basis. In addition, the values could be averagedfor each vector over the course of one week, for example. Averaging mayconsist of a moving average or recursive average depending on timeweighting and memory considerations.

FIG. 3 is a graphical representation of cardiac signals sensed alongmultiple sensing vectors during selection of a sensing vector in amedical device according to one embodiment. As illustrated in FIG. 3,during the vector selection process, the device senses a cardiac signalfor each available sensing vector, using sensing techniques known in theart, such as described, for example, in U.S. patent application Ser. No.14/250,040, incorporated herein by reference in it's entirety, and basedon the result of the sensed signals, ranks the available sensing vectorsand determines one or more desired sensing vectors based on theresulting ranking of sensing vectors 102-106. For example, asillustrated in FIG. 3, according to one embodiment, the device senses anECG signal 100 from each of the available sensing vectors, including ahorizontal sensing vector 102 extending between the housing or can 25and electrode 22, a diagonal sensing vector 104 extending between thehousing or can 25 and electrode 20, and a vertical sensing vector 106extending between electrodes 20 and 22. The device determines a sensedR-wave 108 for each sensing vector 102-106 as occurring when the sensedsignal exceeds a time-dependent self-adjusting sensing threshold 110.

Once the R-wave 108 is sensed, the device sets a vector quality metricdetection window 112 based on the sensed R-wave 108 for each of thesensing vectors 102-106, for determining a vector quality metricassociated with the sensing vectors 102-106. According to an embodiment,the device sets a quality metric detection window 112 to start at astart point 114 located a predetermined distance 116 from the R-wave108, and having a detection window width 118 so as to allow an analysisof the signal 100 to be performed in an expected range of the signal 100where a T-wave of the QRS signal associated with the sensed R-wave 108is likely to occur. For example, the device sets the quality metricdetection window 112 as having a width 118 of approximately 200 ms, witha start point 114 of the quality metric detection window 112 locatedbetween approximately 150-180 milliseconds from the sensed R-wave 108,and the width 118 extending 200 ms, for example, from the detectionwindow start point 114 to a detection window end point 120, i.e., at adistance of approximately 350-380 ms from the detected R-wave 108.According to another embodiment, the width 118 extends 270 ms, forexample, from the detection window start point 114 to a detection windowend point 120, i.e., at a distance of approximately 420-450 ms from thedetected R-wave 108. Once the quality metric detection window 112 isset, the device determines a minimum signal difference 122 between thesensed signal 100 and the sensing threshold 110 within the qualitymetric detection window 112, i.e., the minimum distance extendingbetween the sensed signal 100 and the sensing threshold 110, asdescribed below.

FIG. 4 is a flowchart of a method for selecting one or more sensingvectors according to an exemplary embodiment. As illustrated in FIGS. 3and 4, for each cardiac signal 100 obtained from the respective sensingvectors 102-106, the device obtains the sensed R-wave 108 of the cardiacsignal 100, Block 200, and sets the quality metric detection window 112,Block 202, based on the sensed R-wave 108 for that sensing vector102-106. Once the quality metric detection window 112 is located, thedevice determines the minimum signal difference 122 between the sensedcardiac signal 100 and the sensing threshold 110 within the qualitymetric detection window 112 for each of the sensing vectors, Block 204.The determined minimum signal difference 122 is stored, and the devicedetermines whether the minimum signal difference 122 has been determinedfor a predetermined threshold number of cardiac cycles for each of thesensing vectors 102-106, Block 206. If the minimum signal difference hasnot been determined for the predetermined threshold number of cardiaccycles for each sensing vector 102-106, No in Block 206, the device getsthe next R-wave 108 for each sensing vector 102-106, and the process isrepeated for a next sensed cardiac cycle for each of the sensing vectors102-106. According to one embodiment, the minimum signal difference 122is determined for 15 cardiac cycles, for example.

Once the minimum signal difference 122 has been determined for all ofthe predetermined threshold number of cardiac cycles, Yes in Block 206,the device determines a vector selection metric for each vector 102-106based on the 15 minimum signal differences 122 determined for thatvector, Block 208. For example, according to an embodiment, the devicedetermines the median of the 15 minimum signal differences 122 for eachsensing vector and sets the vector selection metric for that sensingvector equal to the determined median of the associated minimum signaldifferences 122. Once a single vector selection metric is determined foreach of the sensing vectors 102-106 in Block 208, the device ranks thevector selection metrics for the sensing vectors 102-106, Block 210. Forexample, the device ranks the determined vector selection metrics fromhighest to lowest, so that in the example of FIG. 3, the diagonalsensing vector 104 would be ranked highest since the median minimumsignal difference for that vector was 0.84 millivolts, the horizontalsensing vector 102 would be ranked second, since the median minimumsignal difference for that vector is 0.82 millivolts, and the verticalsensing vector 106 would be ranked third, since the median minimumsignal difference for that sensing vector is 0.55 millivolts.

Once the sensing vectors have been ranked in Block 210, the deviceselects the sensing vector(s) to be utilized during subsequent sensingand arrhythmia detection by the device, Block 212. Depending on theamount of time programmed to occur between updating of the sensingvectors 102-106, i.e., an hour, day, week or month, for example, thedevice waits until the next scheduled vector selection determination,Block 214, at which time the vector selection process is repeated.

FIG. 5 is a flowchart of a method for selecting one or more sensingvectors according to another exemplary embodiment. As illustrated inFIGS. 3 and 5, according to another embodiment, for each cardiac signal100 obtained from the respective sensing vectors 102-106, the deviceobtains the sensed R-wave 108 of the cardiac signal 100, Block 300, andsets the quality metric detection window 112, Block 302, based on thesensed R-wave 108 for that sensing vector 102-106, as described above.Once the quality metric detection window 112 is located, the devicedetermines the minimum signal difference 122 between the sensed cardiacsignal 100 and the sensing threshold 110 within the quality metricdetection window 112 for each of the sensing vectors 102-106, Block 304.The determined minimum signal difference 122 is stored, and the devicedetermines whether the minimum signal difference 122 has been determinedfor a predetermined threshold number of cardiac cycles for each sensingvector 102-106, Block 306, i.e., such as 15 cardiac cycles, for example.

If the minimum signal difference 122 has not been determined for thethreshold number of cardiac cycles for each sensing vector 102-106, Noin Block 306, the device determines whether a predetermined timer hasexpired, Block 308. If the timer has not expired, No in Block 308, thedevice gets the next R-wave 108 for each sensing vector 102-106, and theprocess is repeated for a next sensed cardiac cycle for each of thesensing vectors 102-106. According to one embodiment, the timer in Block308 is set as 40 seconds, for example.

In some instances, the device may have not been able to obtain therequired number of minimum signal differences 122 for one or more of thesensing vectors, and therefore if the timer has expired, Yes in Block308, the device determines whether the required number of minimum signaldifferences was obtained for at least 2 of the sensing vectors 102-106,Block 314. If the required number of minimum signal differences was notobtained for at least 2 of the sensing vectors, i.e., for only one ornone of the sensing vectors 102-106, No in Block 314, the devicedetermines no sensing vector selection can be made, Block 310, anddepending on the amount of time programmed to occur between updating ofthe sensing vectors 102-106, i.e., an hour, day, week or month, forexample, the device waits until the next scheduled vector selectiondetermination, Block 312, at which time the vector selection process isrepeated.

If the required number of minimum signal differences was obtained for atleast 2 of the sensing vectors 102-106, Yes in Block 314, the deviceselects those two sensing vectors in Block 320 to be utilized duringsubsequent sensing and arrhythmia detection by the device. As describedabove, depending on the amount of time programmed to occur betweenupdating of the sensing vectors 102-106, i.e., an hour, day, week ormonth, for example, the device waits until the next scheduled vectorselection determination, Block 312, at which time the vector selectionprocess is then repeated.

If the minimum signal difference 122 has been determined for thepredetermined number of cardiac cycles for each sensing vector 102-106,Yes in Block 306, the device determines a vector selection metric foreach vector 102-106 based on the 15 minimum signal differences 122determined for that vector, Block 316. For example, according to anembodiment, the device determines the median of the 15 minimum signaldifferences 122 for each sensing vector and sets the vector selectionmetric for that sensing vector equal to the determined median of theassociated minimum signal differences 122. Once a single vectorselection metric is determined for each of the sensing vectors 102-106in Block 316, the device ranks the vector selection metrics for thesensing vectors 102-106, Block 318. For example, the device ranks thedetermined vector selection metrics from highest to lowest, so that inthe example of FIG. 3, the diagonal sensing vector 104 would be rankedhighest since the median minimum signal difference for that vector was0.84 millivolts, the horizontal sensing vector 102 would be rankedsecond, since the median minimum signal difference for that vector is0.82 millivolts, and the vertical sensing vector 106 would be rankedthird, since the median minimum signal difference for that sensingvector is 0.55 millivolts.

Once the sensing vectors have been ranked in Block 318, the deviceselects the sensing vector(s) to be utilized during subsequent sensingand arrhythmia detection by the device, Block 320. According to anotherembodiment, the results of the ranking may be displayed, such as on aprogrammer, to enable a user to select the sensing vector(s). Dependingon the amount of time programmed to occur between updating of thesensing vectors 102-106, i.e., an hour, day, week or month, for example,the device waits until the next scheduled vector selectiondetermination, Block 312, at which time the vector selection process isrepeated. In addition, according to another embodiment, the user maymanually initiate the vector selection process, so that the device wouldwait until the user input is received, and which point the nextscheduled vector selection process would be repeated.

FIG. 6 is a flowchart of a method for selecting one or more sensingvectors according to another exemplary embodiment. As described above,in some instances, the device may have not been able to obtain therequired number of minimum signal differences 122 for one or more of thesensing vectors. In addition, there may be instances where the minimumsignal difference 122 for one or more of the cardiac cycles in one ormore of the sensing vectors 102-106 is equal to zero when the ECG signalis greater than or equal to the sensing threshold in one or more cardiaccycles or throughout the quality metric sensing window 112. Suchinstances of zero minimum signal differences may likely reflect eitherT-wave oversensing, frequent premature ventricular contractions, highrate sensing (greater than 150 beats per minute) or noise occuring inthe sensing vector.

Therefore, according to one embodiment, as illustrated in FIGS. 3 and 6,for each cardiac signal 100 obtained from the respective sensing vectors102-106, the device obtains the sensed R-wave 108 of the cardiac signal100, Block 400, and sets the quality metric detection window 112, Block402, based on the sensed R-wave 108 for that sensing vector 102-106, asdescribed above. Once the quality metric detection window 112 islocated, the device determines the minimum signal difference 122 betweenthe sensed cardiac signal 100 and the sensing threshold 110 within thequality metric detection window 112 for each of the sensing vectors102-106, Block 404. In addition, the device determines, for each sensingvector 102-106, whether a zero minimum signal difference occurred duringthe detection window 112, Block 416. If a zero minimum signal differenceoccurred during the detection window 112, Yes in Block 416, the R-waveassociated with that vector is discarded, Block 418, and a determinationis made as to whether a timer has expired, as described below.

If a zero minimum signal difference did not occur during the detectionwindow 112, No in Block 416, the device determines whether the minimumsignal difference 122 has been determined for a predetermined thresholdnumber of cardiac cycles, Block 406, i.e., such 15 cardiac cycles, forexample. If the minimum signal difference 122 has not been determinedfor the threshold number of cardiac cycles for each sensing vector102-106, No in Block 406, the device determines whether thepredetermined timer has expired, Block 408. If the timer has notexpired, No in Block 408, the device gets the next R-wave 108 for eachsensing vector 102-106, and the process is repeated for a next sensedcardiac cycle for each of the sensing vectors 102-106. According to oneembodiment, the timer in Block 408 is set as 40 seconds, for example.

If the timer has expired, Yes in Block 408, the device determineswhether the required number of minimum signal differences was obtainedfor at least 2 of the sensing vectors 102-106, Block 414. If therequired number of minimum signal differences was not obtained for atleast 2 of the sensing vectors, i.e., for only one or none of thesensing vectors 102-106, No in Block 414, the device determines nosensing vector selection can be made, Block 410, and depending on theamount of time programmed to occur between updating of the sensingvectors 102-106, i.e., an hour, day, week or month, for example, thedevice waits until the next scheduled vector selection determination,Block 412, at which time the vector selection process is repeated.

If the required number of minimum signal differences was obtained for atleast 2 of the sensing vectors 102-106, Yes in Block 414, the deviceselects those two sensing vectors in Block 320 to be utilized duringsubsequent sensing and arrhythmia detection by the device. As describedabove, depending on the amount of time programmed to occur betweenupdating of the sensing vectors 102-106, i.e., an hour, day, week ormonth, for example, the device waits until the next scheduled vectorselection determination, Block 412, at which time the vector selectionprocess is then repeated.

If the minimum signal difference 122 has been determined for thethreshold number of cardiac cycles for each sensing vector 102-106, Yesin Block 406, the device determines a vector selection metric for eachvector 102-106 based on the 15 minimum signal differences 122 determinedfor that vector, Block 420. For example, according to an embodiment, thedevice determines the median of the 15 minimum signal differences 122for each sensing vector and sets the vector selection metric for thatsensing vector equal to the determined median of the associated minimumsignal differences 122. Once a single vector selection metric isdetermined for each of the sensing vectors 102-106 in Block 420, thedevice ranks the vector selection metrics for the sensing vectors102-106, Block 422. For example, the device ranks the determined vectorselection metrics from highest to lowest, so that in the example of FIG.3, the diagonal sensing vector 104 would be ranked highest since themedian minimum signal difference for that vector was 0.84 millivolts,the horizontal sensing vector 102 would be ranked second, since themedian minimum signal difference for that vector is 0.82 millivolts, andthe vertical sensing vector 106 would be ranked third, since the medianminimum signal difference for that sensing vector is 0.55 millivolts.

FIG. 7 is a graphical representation of cardiac signals sensed alongmultiple sensing vectors during selection of a sensing vector in amedical device according to another embodiment. As illustrated in FIG.7, during the vector selection process, the device senses a cardiacsignal 500 for one or more of the available sensing vector, as describedabove. When the signal 500 exceeds a time-dependent, self-adjustingsensing threshold 510, an R-wave 508 is sensed. Once an R-wave 508 issensed, the device sets a predetermined blanking period 509 and aquality metric window 512 based on the sensed R-wave 508 for each ofsensing vectors 102-106, for determining a vector quality metricassociated with the sensing vectors 102-106. According to oneembodiment, the device sets the blanking period 509 as extending apredetermined period of time starting from the sensed R-wave 508 andextending to a blanking period endpoint 511. For example, the blankingperiod 509 may be set as 150 ms, although any initial setting may beused, typically between 150 and 180 ms. A blanking period adjustmentwindow 513 is set, extending from the blanking period endpoint 511 to ablanking period adjustment end point 514. A width 517 of the blankingperiod adjustment window 513 will be dependent upon the current settingof the blanking period. Since, according to one embodiment, the blankingperiod is typically set between 150 and 180 ms, if the blanking periodwas 150 ms, the blanking period adjustment window 513 would be 30 ms. Inthis way, the width 517 of the blanking period adjustment window 513 isdependent upon the selected blanking period range.

In addition, the device sets the quality metric window 512 to start atthe blanking period adjustment endpoint 514 and having a detectionwindow width 518 so as to allow an analysis of the signal 500 to beperformed in an expected range of the signal 500 where a T-wave of theQRS signal associated with the sensed R-wave 508 is likely to occur. Forexample, the device sets the quality metric detection window 512 ashaving a width 518 of approximately 200 ms, with a start point 515 ofthe quality metric detection window 512 located at the blanking periodendpoint 514 and the width 518 extending 200 ms from the detectionwindow start point 515 to a detection window end point 520, i.e., at adistance of approximately 350 ms from the detected R-wave 508 when theblanking period 509 is set at 150 ms. Once the blanking periodadjustment window 513 and quality metric detection window 512 are set,the device determines a minimum signal difference 519 between the sensedsignal 500 and the sensing threshold 510 within the blanking periodadjustment window 513, and a minimum signal difference 522 between thesensed signal 500 and the sensing threshold 510 within the qualitymetric detection window 512, as described below.

FIG. 8 is a flowchart of a method for selecting one or more sensingvectors according to another exemplary embodiment. As illustrated inFIGS. 7 and 8, for each cardiac signal 500 obtained from the respectivesensing vectors 102-106, the device obtains a sensed R-wave 508 of thecardiac signal 500, Block 600, and sets the blanking period adjustmentwindow 513 and the quality metric detection window 512, Block 602, basedon the sensed R-wave 508 for that sensing vector 102-106. Once theblanking period adjustment window 513 and the quality metric detectionwindow 512 is located, the device determines the minimum signaldifference 519 between the sensed cardiac signal 500 and the sensingthreshold 510 within the blanking period adjustment window 513 and theminimum signal difference 522 between the sensed cardiac signal 500 andthe sensing threshold 510 within the quality metric detection window 512for each of the sensing vectors, Block 604. The determined minimumsignal differences 519 and 522 are stored, and the device determineswhether the minimum signal differences 519 and 522 have been determinedfor a predetermined threshold number of cardiac cycles for each of thesensing vectors 102-106, Block 606.

If the minimum signal differences 519 and 522 have not been determinedfor the predetermined threshold number of cardiac cycles for eachsensing vector 102-106, No in Block 606, the device gets the next R-wave508 for each sensing vector 102-106, and the process is repeated for anext sensed cardiac cycle for each of the sensing vectors 102-106.According to one embodiment, the minimum signal differences 519 and 522are determined for 15 cardiac cycles, for example.

Once the minimum signal differences 519 and 522 have been determined forall of the predetermined threshold number of cardiac cycles, Yes inBlock 606, the device determines a vector selection metric for eachvector 102-106 based on the 15 minimum signal differences 522 determinedfor that vector, Block 608. For example, according to an embodiment, thedevice determines the median of the 15 minimum signal differences 522for each sensing vector and sets the vector selection metric for thatsensing vector equal to the determined median of the associated minimumsignal differences 522. Once a single vector selection metric isdetermined for each of the sensing vectors 102-106 in Block 608, thedevice ranks the vector selection metrics for the sensing vectors102-106, Block 610. For example, the device ranks the determined vectorselection metrics from highest to lowest, so that in the example of FIG.7, the diagonal sensing vector 104 would be ranked highest since themedian minimum signal difference for that vector was 0.86 millivolts,the horizontal sensing vector 102 would be ranked second, since themedian minimum signal difference for that vector is 0.63 millivolts, andthe vertical sensing vector 106 would be ranked third, since the medianminimum signal difference for that sensing vector is 0.32 millivolts.

Once the sensing vectors have been ranked in Block 610, the deviceselects the sensing vector(s) to be utilized during subsequent sensingand arrhythmia detection by the device, Block 612. In addition, thedevice may determine whether the current set blanking period is lessthan a predetermined blanking period threshold corresponding to adesired maximum time period, i.e., 180 ms as described above, Block 614.Depending on the amount of time programmed to occur between updating ofthe sensing vectors 102-106, i.e., an hour, day, week or month, forexample, if the current blanking is not less than the blanking periodthreshold, No in Block 614, the device waits until the next scheduledvector selection determination, Block 622, at which time the vectorselection process is repeated.

If the current blanking is less than the blanking period threshold, Yesin Block 614, the device determines a blanking period adjustment metricfor each vector 102-106 based on the 15 minimum signal differences 519,Block 616. For example, according to an embodiment, the devicedetermines the median of the 15 minimum signal differences 519 for eachsensing vector and sets the blanking period adjustment metric for thatsensing vector equal to the determined median of the associated minimumsignal differences 519. Once a single blanking period adjustment metricis determined for each of the sensing vectors 102-106 in Block 616, thedevice determines whether an adjustment to the blanking period is tomade for the sensing vectors 102-106, Block 618. One reason forincreasing the blanking period is to avoid double counting of R-waves.According to one embodiment, in order to determine whether to adjust theblanking period from the current setting, the device determines, foreach of the two highest ranked vectors from Block 610, whether thecorresponding determined blanking period adjustment metric is less thana proportion of the corresponding determined vector selection metric andless than a predetermined minimum blanking period threshold.

For example, using the results of FIG. 7, where the diagonal sensingvector 104 would be ranked highest since the median minimum signaldifference for that vector was 0.86 millivolts, and the horizontalsensing vector 102 would be ranked second, since the median minimumsignal difference for that vector is 0.63 millivolts, the devicedetermines whether the blanking period adjustment metric determined forthe diagonal sensing vector 104 is both less than one half of the vectorselection metric for that sensing vector, i.e., 0.86 millivolts, andless than 0.05 millivolts, and whether the blanking period adjustmentmetric determined for the horizontal sensing vector 102 is both lessthan one half of the vector selection metric for that sensing vector,i.e., 0.63 millivolts, and less than 0.05 millivolts.

If the blanking period adjustment metric is not less than thepredetermined proportion of one of the two highest ranked vectors or isnot less than the predetermined minimum blanking period threshold, No inBlock 618, the device waits until the next scheduled vector selectiondetermination, Block 622, at which time the vector selection process isrepeated. Depending on the amount of time programmed to occur betweenupdating of the sensing vectors 102-106, the next scheduled updating ofthe vectors may occur in an hour, a day, a week or a month, for example.

If the blanking period adjustment metric is less than the predeterminedproportion of one of the two highest ranked vectors and less than apredetermined minimum blanking period threshold, Yes in Block 618, theblanking period is updated, Block 620. For example, the blanking periodmay be increased to 180 ms, or may be increased by a predeterminedamount, such as 10 ms. In addition, according to another embodiment,rather than automatically increasing the blanking period, the device maygenerate an alarm or other stored indication to indicate to theattending medical personnel that the blanking period should be increasedso that the increase in the blanking period can be adjusted manually,using a programmer or other input device.

It is understood that in addition to the three sensing vectors 102-16described above, optionally, a virtual signal (i.e., a mathematicalcombination of two vectors) may also be utilized in addition to, thusutilizing more than three sensing vectors, or in place of the sensingvectors described. For example, the device may generate a virtual vectorsignal as described in U.S. Pat. No. 6,505,067 “System and Method forDeriving Virtual ECG or EGM Signal” to Lee, et al; both patentsincorporated herein by reference in their entireties. In addition,vector selection may be selected by the patient's physician andprogrammed via a telemetry link from a programmer.

In addition, while the use of a minimum signal difference is described,the device may utilize other selection criteria for ranking vectors. Forexample, according one embodiment, the device may determine, for eachvector, a maximum signal amplitude within the detection window for eachR-wave, determine the difference between the maximum amplitude and thesensing threshold for each of the maximum amplitudes, and determine amedian maximum amplitude difference for each sensing vector over 15cardiac cycles. The device would then select the vector(s) having thegreatest median maximum amplitude difference as the sensing vector(s) tobe utilized during subsequent sensing and arrhythmia detection by thedevice. According to another embodiment, the maximum amplitude in thequality metric window may be subtracted from the maximum R-waveamplitude, or the maximum amplitude in the quality metric window issubtracted from the sense threshold at the time of the maximumamplitude.

Thus, a method and apparatus for selecting a sensing vectorconfiguration in a medical device have been presented in the foregoingdescription with reference to specific embodiments. It is appreciatedthat various modifications to the referenced embodiments may be madewithout departing from the scope of the disclosure as set forth in thefollowing claims.

We claim:
 1. A method of determining sensing vectors in a medicaldevice, comprising: sensing cardiac signals from a plurality ofelectrodes, the plurality of electrodes forming a plurality of sensingvectors; setting a blanking period and a blanking period adjustmentwindow for the plurality of sensing vectors in response to the sensedcardiac signals; determining first signal differences during theblanking period adjustment window; and adjusting the blanking period inresponse to the determined first signal differences.
 2. The method ofclaim 1, wherein adjusting the blanking period comprises increasing theblanking period to extend within the blanking period adjustment window.3. The method of claim 1, further comprising sensing an R-wave inresponse to the cardiac signal exceeding a sensing threshold, whereindetermining first signal differences comprises: determining signaldifferences between the cardiac signal sensed during the blanking periodadjustment window and the sensing threshold; and determining a minimumsignal difference of the determined signal differences.
 4. The method ofclaim 1, further comprising: setting a quality metric signal extendingsubsequent to the blanking period; determining second signal differencesduring the quality metric window; ranking sensing vectors of theplurality of sensing vectors in response to the determined second signaldifferences; and selecting one or more sensing vectors of the pluralityof sensing vectors in response to the determined rankings.
 5. The methodof claim 4, further comprising sensing an R-wave in response to thecardiac signal exceeding a sensing threshold, wherein determining firstsignal differences and determining second signal differences comprises:determining first signal differences between the cardiac signal sensedduring the blanking period adjustment window and the sensing threshold;determining second signal differences between the cardiac signal sensedduring the quality metric window and the sensing threshold; anddetermining a first minimum signal difference of the determined firstsignal differences and a second minimum signal difference of thedetermined second signal differences.
 6. The method of claim 5, furthercomprising: determining a vector selection metric for each of theplurality of sensing vectors in response to the determined second signaldifferences; comparing the determined vector selection metrics from eachof the plurality of sensing vectors; and determining a first rankedsensing vector having a first vector selection metric, a second rankedsensing vector having a second vector selection metric less than thefirst vector selection metric, and a third ranked sensing vector havinga third selection metric less than the second selection metric inresponse to comparing the determined vector selection metrics, whereinselecting the one or more sensing vectors comprises selecting the firstranked sensing vector and the second ranked sensing vector.
 7. Themethod of claim 6, further comprising: determining a blanking adjustmentmetric for each of the plurality of sensing vectors in response to thedetermined first signal differences; comparing a first blanking periodadjustment metric determined for the first ranked sensing vector to thefirst vector selection metric and a second blanking period adjustmentmetric determined for the second ranked sensing vector to the secondvector selection metric; and adjusting the blanking period in responseto comparing the first and second blanking period adjustment metrics. 8.The method of claim 7, further comprising: determining whether the firstblanking period adjustment metric is less than a proportion of the firstvector selection metric; determining whether the first blanking periodadjustment metric is less than a minimum blanking period threshold;determining whether the second blanking period adjustment metric is lessthan a proportion of the second vector selection metric; and determiningwhether the second blanking period adjustment metric is less than theminimum blanking period threshold.
 9. The method of claim 8, wherein theproportion is one half and the minimum blanking period threshold is 0.05millivolts.
 10. The method of claim 9, wherein adjusting the blankingperiod comprises increasing the blanking period to extend within theblanking period adjustment window.
 11. A medical device for determiningsensing vectors, comprising: a plurality of electrodes capable offorming a plurality of sensing vectors for sensing cardiac signals; anda processor configured to sense cardiac signals from the plurality ofelectrodes, set a blanking period and a blanking period adjustmentwindow for the plurality of sensing vectors, determine first signaldifferences during the blanking period adjustment window, and adjust theblanking period in response to the determined first signal differences.12. The medical device of claim 11, wherein adjusting the blankingperiod comprises increasing the blanking period to extend within theblanking period adjustment window.
 13. The medical device of claim 11,wherein the processor is further configured to sense an R-wave inresponse to the cardiac signal exceeding a sensing threshold, determinesignal differences between the cardiac signal sensed during the blankingperiod adjustment window and the sensing threshold, and determine aminimum signal difference of the determined signal differences.
 14. Themedical device of claim 11, wherein the processor is further configuredto set a quality metric signal extending subsequent to the blankingperiod, determine second signal differences during the quality metricwindow, rank sensing vectors of the plurality of sensing vectors inresponse to the determined second signal differences, and select one ormore sensing vectors of the plurality of sensing vectors in response tothe determined rankings.
 15. The medical device of claim 14, wherein theprocessor is further configured to sense an R-wave in response to thecardiac signal exceeding a sensing threshold, determine first signaldifferences between the cardiac signal sensed during the blanking periodadjustment window and the sensing threshold, determine second signaldifferences between the cardiac signal sensed during the quality metricwindow and the sensing threshold, and determine a first minimum signaldifference of the determined first signal differences and a secondminimum signal difference of the determined second signal differences.16. The medical device of claim 15, wherein the processor is furtherconfigured to determine a vector selection metric for each of theplurality of sensing vectors in response to the determined second signaldifferences, compare the determined vector selection metrics from eachof the plurality of sensing vectors, and determine a first rankedsensing vector having a first vector selection metric, a second rankedsensing vector having a second vector selection metric less than thefirst vector selection metric, and a third ranked sensing vector havinga third selection metric less than the second selection metric inresponse to comparing the determined vector selection metrics, whereinselecting the one or more sensing vectors comprises selecting the firstranked sensing vector and the second ranked sensing vector.
 17. Themedical device of claim 16, wherein the processor is further configuredto determine a blanking adjustment metric for each of the plurality ofsensing vectors in response to the determined first signal differences,compare a first blanking period adjustment metric determined for thefirst ranked sensing vector to the first vector selection metric and asecond blanking period adjustment metric determined for the secondranked sensing vector to the second vector selection metric, and adjustthe blanking period in response to comparing the first and secondblanking period adjustment metrics.
 18. The medical device of claim 17,wherein the processor is further configured to determine whether thefirst blanking period adjustment metric is less than a proportion of thefirst vector selection metric, determine whether the first blankingperiod adjustment metric is less than a minimum blanking periodthreshold, determine whether the second blanking period adjustmentmetric is less than a proportion of the second vector selection metric,and determine whether the second blanking period adjustment metric isless than the minimum blanking period threshold.
 19. The medical deviceof claim 18, wherein the proportion is one half and the minimum blankingperiod threshold is 0.05 millivolts.
 20. The medical device of claim 19,wherein adjusting the blanking period comprises increasing the blankingperiod to extend within the blanking period adjustment window.
 21. Anon-transitory, computer-readable storage medium storing instructionsfor causing a processor included in a medical device to perform a methodfor determining sensing vectors, the method comprising: sensing cardiacsignals from a plurality of electrodes, the plurality of electrodesforming a plurality of sensing vectors; setting a blanking period and ablanking period adjustment window for the plurality of sensing vectorsin response to the sensed cardiac signals; determining first signaldifferences during the blanking period adjustment window; and adjustingthe blanking period in response to the determined first signaldifferences.